The invention relates generally to ignition systems. More particularly, the invention relates to ignition systems for combustion engines, especially coil ignition systems. The ignition systems of special interest are those of the type having pre-ignition gaps connected in series in the high-tension lines thereof.
In the conventional ignition systems, without pre-ignition gaps, the spark plug voltage builds up relatively slowly until a potential difference, i.e., voltage, equal to the sparking voltage is set up between the electrodes of the spark plugs. At this time, a spark is generated between the electrodes of the spark plugs, i.e., across the spark plug gap. Normally, the spark plugs are provided with a ceramic jacket which serves as an insulator. When that portion of the ceramic jacket adjacent the electrodes of the spark plug becomes coated with a layer of carbon black and/or lead residues, or if this portion of the ceramic jacket is wet or coated with oil, then part of the charge or energy delivered to the spark plug by the coil of the ignition system flows off along the coating on the ceramic jacket. The charge flowing along this coating does not participate in setting up a potential difference between the electrodes of the spark plug so that this charge is effectively wasted. When the coating on the ceramic jacket becomes sufficiently heavy, so much of the charge delivered to the spark plug leaks away, i.e., flows off, that the voltage between the electrodes required for generating a spark can no longer be attained.
This susceptibility to leakage of the ignition systems for combustion engines may, in known manner, be greatly reduced by connecting pre-ignition gaps in series in the high-tension leads of the ignition system. A pre-ignition gap for this purpose may be connected in the circuit immediately upstream of the spark plug or may be built into the spark plug itself. In order to serve its purpose, the pre-ignition gap should have a breakdown voltage, i.e., voltage at which sparking occurs therein, which is greater than that of the spark plug. Since the pre-ignition gap is located upstream of the spark plug, i.e., upstream of the spark plug gap, no sparking across the spark plug gap can occur until sparking has first occurred across the pre-ignition gap, that is, the generation of a spark across the pre-ignition gap "completes" the circuit to the spark plug. Thus, since the sparking voltage of the pre-ignition gap is greater than that of the spark plug, a relatively high voltage will exist across the spark plug gap at least at the instant that sparking occurs across the pre-ignition gap. As a result, a spark will be generated across the spark plug gap even though the ceramic insulating jacket of the spark plug may be coated with a layer of dirt. The reason is that the leakage of charge along the coating on the ceramic jacket of the spark plug is negligibly small compared to the high voltage which rapidly comes into being across the spark plug gap at the instant that sparking occurs across the pre-ignition gap. In other words, when sparking occurs across the pre-ignition gap, the voltage across the spark plug gap builds up almost instantaneously so that practially no time exists for leakage of charge to occur. This is in contrast to ignition systems without pre-ignition gaps where the voltage across the spark plug gap builds up slowly so that ample time exists for charge to leak away.
There has been much research directed to the problem of producing suitable pre-ignition gaps for connection into ignition systems of both the type which may be built into the bore of the spark plug insulation and the type which may be in the form of separate elements located in a plug connector which is then plugged into the spark plug. The focus has been on providing pre-ignition gaps which are suitable for the ignition systems currently used in combustion engines and which will insure ignition even for lean fuel-air mixtures. Some current pre-ignition gaps are disclosed, for example, in the U.S. Pat. Nos. 2,505,150 and 2,965,779. The known pre-ignition gaps are, however, very expensive because the cost of producing electrodes which are low in sputtering is high and, in addition, the known electrodes impose strict requirements as regards the purity of the gas surrounding them in the region of sparking. One of the reasons that the known electrodes are high in cost is that, until now, these were provided with coverings or coatings consisting of oxinitrides or nitrides. The production of such oxinitrides and nitrides and the processes involved in coating the electrodes with them require additional and expensive production steps. Furthermore, the known pre-ignition gaps are not fully satisfactory with respect to the stability of their sparking voltage and with respect to their frequency dependence. Thus, in the known pre-ignition gaps, the sparking voltage fluctuates over a wide range. If the sparking voltage of the pre-ignition gap is too high, then it may exceed the voltage which can be supplied by the ignition system with the result that no sparking whatsoever will occur (even with new spark plugs). On the other hand, if the sparking voltage of the pre-ignition gap is too low, the pre-ignition gap loses its effectiveness since sparking across it will occur at a relatively low voltage. The result of this is that, at the instant that sparking occurs across the pre-ignition gap, the voltage across the spark plug gap will also be relatively low so that the loss of charge along the coating on the ceramic jacket of the spark plug becomes significant again and no sparking will occur across the spark plug gap. Insofar as the frequency dependence of the pre-ignition gap is concerned, the sparking voltage of the known pre-ignition gaps falls off greatly at higher frequencies so that, again, the pre-ignition gap loses its effectiveness.